The versatility of polyamide-polyamine oligomers has led to their use in applications ranging from structural components to hot-melt adhesives. Part of this versatility may be attributed to the different processes of preparation and the many different polymeric backbones, all of which bestow the product with its specific properties. Evidently, the oligomers properties may be adjusted by manipulations of the components in the polycondensation or polyaddition reactions employed for their preparation.
Organic polymers are generally classified according to their structure as either linear, branched or cross-linked. In the case of linear polymers, the repeating units, also referred to as n-mers, are bivalent and are connected one to another in a linear sequence. The number of mers in a polymer, (n), varies and depends on the ratio between the reactants in the polycondensation/polyaddition reaction. In the case of branched polymers, at least some of the mers possess a valency greater than 2, such that the mers may be connected in a nonlinear sequence. The term "branching" usually indicates that the individual molecular units of the branches are discrete from the polymer backbone, yet have the same chemical constitution as the polymer backbone.
The simplest type of branching known in the art is the comb branching, wherein the branches are uniformly and regularly distributed on the polymer backbone, or irregular, wherein the branches are distributed in non-uniform or random fashion on the polymer backbone. Another type of branching is referred to as the cross-linked or network polymer, wherein the polymer chains are connected via tetravalent compounds. These types of polymer are widely used for curing.
The implementations of the different polyamides vary, depending on their structure and valency (reactive groups composition). In particular, polyamine-polyamide oligomers may be used as epoxy hardeners in the preparation of thermosetting compositions such as adhesives, e.g. coatings, as lacquers, sealants, and putty adhesives. Alternatively, the polyamine-polyamide oligomers may be used as thermoplastic hot-melt adhesives for metals, wood or concrete; as adhesion promoters for polyamides or polyvinylchloride plastics (PVC); for preparing polymer-coated cellophane and in the preparation of aluminum foil; as alcohol-soluble binding substances for the preparation of printing ink compositions; as well as in non-ionic softening agents or as anti-static agents.
Polyamide resins are mainly prepared by the polycondensation between fatty acid monomers and polyamines. Presently available polycondensation reactions result in the formation of low molecular weight quasi-linear oligomers (molecular weight of between 1,000 and 5,000) and require a stoichiometric excess amount of the di- or polyamine reactant.
The low molecular weight oligomers, which are usually in a liquid state at room temperature, are suitable as epoxy hardeners for glues, varnish-paints and putty materials. Since the physical properties of a resin, such as its strength, flexibility and adhesion properties, correlate with the resin's molecular weight, low molecular weight quasi-linear polyamine-polyamide oligomers may be disadvantageous for many applications. In addition, any change in the liquefaction temperature is significant, since it may result in destruction of the resin and loss of essential properties required for its processing.
To date, attempts to raise the molecular weight of branched oligomers failed, since they led to the decrease in their solubility in standard solvents and to the increase of their softening point (the temperature range in which the resin liquefies).
U.S. Pat. No. 5,756,596 relates to a process for increasing the molecular weight of polyamides, essentially without cross-linking, which process comprises blending the polyamide with a polyfunctional epoxy resin and a satirically hindered hydroxyphenylalkylphosphoric acid ester or half-ester and heating the blend to above the melting point (or the glass transition temperature) of the polyamide, in the absence of a catalyst of the type used in the polymerization of polyamides. The method described is advantageous only when using high molecular weight polyamides. In case low molecular polyamides are employed, highly branched polyamide-epoxy adducts are obtained, which are not compatible with epoxy resins.
German Patent No. DE 2,759,313 relates to polyaminoamide-epoxy adducts obtained by reacting an epoxy compound with an excess of an end-capped polyaminoamide compound comprising free amino groups, or by reacting such a polyaminoamide with an excess of the epoxy compound, the reactions being carried out in a non-aromatic solvent. Two adducts were obtained, the first contained linear or sparsely branched polymers, which, as already detailed above, are of disadvantageous, whereas the second adduct consisted of epoxy residues, thus rendering it unstable and unsuitable for use in the preparation dendripolyamides.
Japanese Patent No. 10 07,791 (98 07,791) relates to an adduct of epoxy resin with polyoxyalkylene amine used in the preparation of printing inks. The adduct being mainly linear is not suitable for the preparation of dendripolyamides.
Recently, a new group of polyamides, the dendripolyamides (highly branched polyamide oligomers, also referred to as dendrimers) has been developed [Newcome GR, et al., Dendric Molecules. Concepts, Synthesis and Perspectives. (1996) Ed. VCH Weinheim; Aoi K, et al., (1997) Macromol. Rapid. Commun. 18(10):945; Evenson SA, et al., (1997) Adv. Mater 9(14):1097]. Dendripolymers, by definition, exhibit higher concentrations of functional groups per nucleus, which renders them more active for their intended purposes.
`Star structured` polymers, developed more recently, are a class of dendripolyamides in which the individual branches radiate out from a core/nucleus. The star-branched polymeric adducts offer several advantages over the other linear or sparsely branched polymers. In addition to the higher concentration of functional groups per nucleus, star-branched polymers are often less sensitive to degradation. Such features become prominent in the manufacture of paints or in enhanced oil recovery. Furthermore, star-branched polymers, have relatively low intrinsic viscosity, even at high molecular weight.
To date, dendripolymers are prepared from a multifunctional core compound and an excess of a suitable reactant. The resulting new multifunctional adduct is then further reacted with the excess of the reactant to obtain the branched product.
U.S. Pat. No. 5,760,142 relates to epoxide-amine dendrimers synthesized by a repetitive and step-wise addition reaction of epoxides comprising functional moieties which are available for conversion into amine groups followed by reaction of these groups to primary amino moieties. The amino terminated dendrimers are reacted with (2,3-epoxypropoxy)methacrylate, a monoepoxide and/or a monoisocyanante. The methacrylate terminated dendrimers are polymerizable using redox initiators and/or photoinitiators. The resulting dendrimers are describes as showing a very low volume shrinkage from that of the starting materials, on the order of less than about 5% by volume. These dendrimers have very limited uses and are particularly useful as polymerizable hot-curing materials. Furthermore, the process described is a complex multistage technology, which renders its products highly expensive.
Dendrimers may be used in a variety of applications, for example, as emulsifiers for oil/water emulsions; as viscosity modifying agents in aqueous formulations such as paints; as wet-strength agents in the manufacture of paper; as high efficiency proton scavengers; as components of calibration standards for electron microscopy; and in preparing size-selective membranes. When compared to linear or sparsely branched polyamides, the dendrimers exhibit improved adhesion to a variety of substrates, and improved flexibility and stability. Nonetheless, dendripolyamides prepared by the methodology described above, which are mostly of low molecular weight, are incompatible with most epoxy resins, and thus are disadvantageous.
Epoxy glues, which are one of many implementations for the dendripolyamides, are widely used both domestically and in various fields of modem industry. Such glues are prepared in the form of two multi-component parts that may be generally defined as part A and Part B. Part A commonly contains an epoxy resin and is compatible with additives. An amine hardener (usually a polyamide resin), a curing catalyst, and some special additives form part B. The production of epoxy glues or any other application involving epoxy compounds, in the form of two parts, is profitable for the manufacturer and the consumer. The two parts of the glue, being kept separately, have a long shelf life. In addition, the exact amount required for curing can be mixed only when needed.
Epoxy-polyamide mixtures are exposed to aggressive media and organic solvents. Presently, such mixtures are cured slowly at room temperatures and the gel time for a thin layer of glue is usually 90 to 120 minutes. However, 7 to 10 days are required to complete curing. In addition, the grasping time for such compositions may be between 5 to 30 minutes, via the addition of active cure catalysts. However, by accelerating the curing process, the durability, adhesion and other essential features of the cured material are reduced. Thus, there exists a need to develop a fast-grasping, but durable, curing agents, which is an aim of the present invention.